Background

Concern about carbon dioxide emissions and world hydrocarbon fuel reserves means that there is considerable interest in technologies that reduce fuel consumption for passenger cars. In the area of vehicle design, body weight is the most important target for improvement, as a reduction in the weight of a vehicle’s body means that a smaller engine, and a lighter drive train and assembly can be used. This ‘benign spiral’ leads to further mass reductions, so much so that various studies have indicated a potential for savings of up to 65% by using carbon fibre composites instead of steel wherever possible.

Racing Cars versus Passenger Cars

The Aero-Stable Carbon Car (ASCC) programme has been investigating the limitations to maximising fuel economy in a lightweight car manufactured using carbon fibre composites (CFC). Current lightweight composite vehicles, such as racing cars, use a monocoque stressed-skin design for both weight and manufacturing cost reasons. However, for passenger cars with large ‘cut-out’ areas for access, the approach of using a space-frame supporting fairing panels offers the opportunity for a more efficient structure compared to the monocoque approach. It also offers the potential to incorporate localised loads, such as those from the suspension, engine and door mountings, seat and seat belts, more easily than with a thin-section stressed-skin approach.

Manufacturing Space-Frame Using a Carbon Fibre Composite Material

Manufacturing the space-frame using carbon fibre composite materials provides a very lightweight structure. But using current manufacturing techniques, the labour cost for bonding sections together and material lay-up limits the use of a framework approach for all but the most expensive niche vehicles.

Fabrication Process and weight Savings

In response to this, a novel design and materials approach was conceived and developed. The approach uses a novel form of textile preform, laminated to form a single-piece integrated frame structure. Lightweight panels are bonded to the assembled frame after systems fitting. This approach results in a total bodyweight of about 125kg, which compares to around 320kg for a similar-sized steel car.

Collaborators and Project Objectives

The Cranfield University Centre for Lightweight Composites, in collaboration with Lotus Engineering (CAD and body pattern manufacture), Cranfield Impact Centre (impact considerations), Tenax Fibers (carbon fibre and preforming advice), Vantico (epoxy resin, tooling materials and adhesives) and BT1 Europe (carbon fibre fabrics), worked on the design and development of a lightweight composite body for a medium‑sized (Ford Focussized) car demonstrator. The project objective was to design and manufacture a carbon fibre composite structure for the ASCC at minimum weight, while providing greater stiffness in all aspects than for current steel bodies.

Materials Developments

The application of carbon or epoxy composites to a lightweight primary structure is not new, but the process has to date been employed exclusively within the prestige sports and racing car industry. An example of this is the McLaren Fl, the structure of which is reported to have required well over 1,000 man-hours of skilled labour to mould the composite components. The transfer of aerospace composites technology has been shown to provide very effective structures although at an unaffordable cost for consumer markets.

Reasons for High Manufacturing Costs of Composite Car Bodies

The very high manufacturing costs of lightweight composite car structures is principally due to three factors:

the high cost of raw materials, both in the use of pre-impregnated fabrics and the very high waste level in laminating complex shape components

The high labour cost required to manufacture weight-optimised components, which need careful draping and alignment of very thin (typically 0.4mm) layers with the thickness tailored to suit load distribution

Very high cycle times for both lay-up and resin curing, hence low production rates from each tool set.

Manufacturing Composite Car Bodies on a Commercial Scale

A suitable manufacturing process for higher volume production (between 1,000 and 10,000 units per annum) of automotive primary structure requires lower cost raw material, use of automation for reinforcement application and impregnation, lower process cycle time and moulded surfaces that require no hand finishing.

Composite Car bodies in Crash Situations

Another current issue for composite car bodies is the insufficient experience of impact behaviour other than for small racing car monocoques. This results from the complex, non-plastic failure of the material, which is difficult to model or predict. Any structure for an automotive application needs to satisfy a number of performance criteria, namely:

Suitable torsional rigidity for ‘regular’ driving

Sufficient stiffness and strength to protect the occupant in the event of a low-speed (<30mph) collision

• progressive and controlled failure of the structure in order to reduce the risk of injury in a highspeed (>30mph) collision.

Typically, the first two of these criteria can be easily met by an advanced composite structure. The third requirement is more difficult to achieve, as the materials tend to behave in a linear-elastic way until failure, which is then instant and catastrophic, leaving minimal residual strength. The area of impact performance of lightweight carbon fibre structures justifies extensive investigation, since carbon fibre composites can provide exceptionally high levels of crash energy absorption if structures are engineered effectively.

Development of a Novel Materials and Manufacturing Process

As a result of the need for a lightweight, low-cost crashworthy structure, a completely novel materials and process concept was conceived and developed. A lightweight single-piece composite structural framework and simple, lightweight bonded panels replace the conventional moulding and assembly of complex shape-stiffened panels. This approach offers several potential manufacturing and performance advantages over the conventional approach, including:

Very substantial materials cost reduction through the use of low-cost textile platforms and liquid resin

Automation of reinforcement pre-form manufacture, application to mould tools and impregnation process and hence very substantial reductions in the labour costs of moulding

Rapid and low labour cost vehicle body assembly with minimal fixtures

Improved crashworthiness.

Difficulties Associated with the Novel Process

However, this approach presented some difficult manufacturing technology challenges since frameworks have complex geometry and joining of composite primary structures is a very labour-intensive process.

The Design of the Composite Passenger Car

The monocoque approach having been discarded, a more efficient design that does not need to transfer large loads through panel joints, is to use a very stiff framework of complex shaped beams and struts, covered by thin panels, bonded using low stiffness adhesives. This approach also offers benefits in vehicle assembly and fitting, since loading and attachment points can be provided on the framework and the panels can be attached near the end of the process to provide clear access through frame apertures.

However, realisation of this design concept was difficult, since current material forms and processing solutions are developed for thin skins and not suited to framework structures.

Composite Framework Design

The resultant framework design comprises a single highly-integrated moulding. This defines the outer shape of the passenger compartment. The majority of the side impact strength is provided by the sill sections, which are up to 300mm deep. The front suspension is attached to the space frame via a bolted aluminium sub-frame, so protecting it from damage during low-level collisions. The engine and rear suspension will be attached to the space frame via a rear bulkhead and a steel sub frame.

Use of Hollow and Foam-Cored Members

The only appropriate structural approach for this framework is through the use of hollow or foam-cored beams connected by very stiff joints. Attempts to engineer carbon fibre composite framework components, such as bicycle frames, has resulted in extremely high labour costs. This results from having to integrate beam-ends at joints or produce and bond complex jointing elements. To overcome this problem, so as to enable the volume production of complex framework structures, a novel materials and design approach was conceived and subsequently patented.

The Patented Process

A conventional beam comprising a fabric of large diameter braid wrapping or enclosing a lightweight foam core is replaced by an array of foam cores, each with a braided carbon fibre sleeve. This ‘biomimetic’ type assembly is impregnated and bonded using a very low viscosity, tough two-part epoxy There are three manufacturing cost advantages that result from using this approach:

1.Through the use of a continuous feedstock applied in a series of foam-filled braided tubes, rather than manipulation of large pieces of fabric, deposition into a mould tool has the potential to be automated.

2.The use of a narrow conformable sleeve also allows joints to be formed by taking the feedstock around curves into connected sections.

3.The avoidance of cutting fabrics to conform to complex shapes and join beams results in a very low level of reinforcement waste. This should be in the order of around 2%, compared to a minimum of 30% for conventional approaches.

Determining Framework Section Sizes

Each section of the framework was manufactured from different configurations of cored sleeves, the number and their arrangement around the joints determined by the required load transfer. To establish an understanding of joint stiffness, a detailed experimental programme was carried out to establish design and manufacturing details for generic ‘T’ piece structures. The parameters of braid style, tow size, wall thickness, array arrangement and joint impregnation process conditions were all examined, and established the need to provide additional material at the surface of the ‘T’ intersection to locally thicken the surface wall thickness.

Panel Manufacture

Approaches to Panel Manufacture

For the panel manufacture, several approaches were investigated. These included multi-axial LIBA-type fabrics, using high-strength type carbon fibres from BTI Europe with Vantico LY 564 resin and HY 2962 OE curing agent, SP Systems’ SPRINT fabric, resin film, syntactic foam and surfacing layer one ply sandwich fabric and Hexcel Composites’ pseudo-thermoplastic prepreg. The roof, floor and rear bulkhead use a sandwich structure comprising one layer of multi-axial fabric each side of a toughened PVC foam core.

Panel Moulding and Fixation

The panels are moulded using a resin infusion process and the prepreg material panels by conventional vacuum bag / oven curing. The requirement to be able to remove any panels without damage to the frame, combined with a caution about bonding preparation for structural joints, resulted in the decision to use ductile adhesives. All of the panels, including the roof, floor and bulkhead, will be bonded using a polyurethane adhesive similar to those used in current windscreen fitting.

Space Frame Strength and Crash Resistance

The project is due for completion at the end of November. The resultant space frame is expected to have a weight of 92kg and an associated torsional rigidity of around 15,00ONm/degree. For crash energy absorption and reduction of crash deceleration rate, two frontal impact systems will be used, in addition to an aluminium subframe attaching to the front suspension, a dedicated crash member will be attached to this sub frame. Consequently, the space frame and occupants should not be subjected to a critical peak load. Side impact crash safety is provided through the very deep sections in the sill areas of the framework. These will deform progressively through the use of thin-walled tubular arrays, yet provide extremely high stiffness and strength to avoid catastrophic failure during the crash.

Summary

In the finished car the frame, panels and doors will have a weight of around 140kg, and a total kerb weight of around 570kg. The project also established a novel design and manufacturing technology for carbon fibre composite car bodies, expected to be viable from a manufacturing perspective for producing up to 20,000 cars per annum. The cycle time limitation is the resin impregnation and curing time, so annual production volumes up to 50,000 cars per annum would be possible using a different matrix of polymers and impregnation/curing technology with greater tooling investment.